The invention relates to a thermal management system for the processing of fuel for fuel cells.
Fuel cells are a leading alternate fuel powerplant candidates for both portable and stationary electrical power generation. A fuel cell is an electrochemical energy converter consisting of two electrodes which sandwich an electrolyte. In one form being developed for both portable and stationary applications, an ion-conducting polymer electrolyte membrane (PEM) is disposed between two electrode layers to form a membrane electrode assembly (MEA). The MEA is typically porous and electrically conductive to promote the desired electrochemical reaction from two reactants. One reactant, oxygen or air, passes over one electrode and hydrogen, the other reactant, passes over the other electrode to produce electricity, water and heat. Typical PEM fuel cells with membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994 and assigned to the General Motors Corporation.
For vehicular applications, it is desirable to use a liquid fuel such as a low molecular weight alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the fuel for the vehicle because of the ease of onboard storage of liquid fuels and the existence of a nationwide infrastructure for supplying liquid fuels. However, liquid fuels must be dissociated to release their hydrogen content from the liquid fuel prior to use in a fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, also known as a reformer, that in conjunction with thermal energy and a suitable catalyst, yields a reformate gas including N2, H2O, CO2, H2 and CO.
The heat required to produce sufficient hydrogen varies with the energy demand required by the fuel cell system at any given moment in time. Accordingly, the heating system for the reformer must be capable of operating over a wide range of energy output. Heating a reformer with heat generated externally is generally known in the prior art. One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh. The reformate exiting the reformer, however, may contain undesirably high concentrations of carbon monoxide (CO) most of which must be removed (i.e., to a concentration of less than about 50 ppm) to prevent poisoning of the fuel cell""s anode.
It is known that the CO level of the reformate/effluent exiting a reformer can be reduced by utilizing a well-known xe2x80x9cwater gas shiftxe2x80x9d (WGS) reaction where water (i.e., in the form of steam) is added to the reformate/effluent exiting the reformer in the presence of a suitable catalyst. This lowers the carbon monoxide content of the reformate/effluent gas.
However, some CO (i.e., about 0.5 mole % or more) still survives the shift reaction. Hence, shift reactor effluent gases include hydrogen, carbon dioxide, water and carbon monoxide. If the shift reaction is not sufficient to reduce the CO content of the reformate to a satisfactory level (i.e., to below about 50 ppm), it may be necessary to further remove carbon monoxide from the hydrogen-rich reformate stream exiting the shift reactor prior to supplying the effluent gas to the fuel cell. It is known to further reduce the CO content of H2-rich reformate gas exiting the shift reactor by a preferential oxidation or PrOx reaction effected in a suitable reactor operated at temperatures which promote the preferential oxidation of the CO by air in the presence of the H2, but without consuming/oxidizing substantial quantities of the H2 or triggering the so-called xe2x80x9creverse water gas shiftxe2x80x9d (RWGS) reaction.
The preferential oxidation process is described in a paper entitled xe2x80x9cMethanol Fuel Processing for Low Temperature Fuel Cellsxe2x80x9d published in the Program and Abstracts of the 1988 Fuel Cell Seminar, Oct. 23-26, 1988, Long Beach, Calif., and in U.S. Pat. No. 5,271,916, issued to Vanderborgh et. al. Preferential oxidation reactors may be either adiabatic (i.e. where the temperature of the reactor is allowed to rise during oxidation of the CO) or isothermal (i.e. where the temperature the reactor is maintained substantially constant during oxidation of the CO). The adiabatic preferential oxidation process is sometimes effected by means of a number of sequential stages, which progressively reduce the CO content in stages, and requires careful temperature control, to prevent the reverse water gas shift reaction which counterproductively consumes H2 and produces more CO. The isothermal process can effect the same CO reduction as the adiabatic process, but in fewer stages and without concern for the reverse water gas shift reaction if the reactor temperature can be kept low enough, and O2 depletion near the exit of the reactor unit can be avoided.
One known isothermal reactor is essentially a catalyzed heat exchanger having a thermally conductive barrier or wall that separates the heat exchanger into a first channel through which the H2-rich gas to be decontaminated (i.e. CO removed) passes, and a second channel through which a coolant flows to maintain the temperature of the reactor substantially constant within a defined working range. The barrier wall has a catalyzed first surface confronting the first channel for promoting the CO+O2 reaction, and an uncatalyzed second surface confronting the second channel for contacting the coolant therein to extract heat from the catalyzed first surface through the barrier. Therefore, it has been found that the proper control of the fuel processor for fuel cells requires the thermal management of the water gas shift and the preferential oxidation reactors such that the reactors (primarily WGS and PrOx) are operated within their preferred temperature ranges. This means removing heat from the reformate stream entering the water gas shift and preferential oxidation reactors and in some cases removing the heat of reaction within the reactors (by means of a catalyzed heat exchanger).
Conventional fuel processor systems have little or no thermal management. One system uses high temperature oil to remove the heat rejected by the preferential oxidation reactor and uses an air-to-oil heat exchanger to reject this heat to the ambient environment. Another system utilizes the heat from the reactors and heat exchangers with high temperature oil. Such systems require additional hardware, add an additional large thermal mass, are complex and add volume to the fuel processor, as well as additional control and maintenance issues.
Therefore, there is a need for a fuel processor thermal management system that does not add additional mass, complexity and volume to the fuel cell thermal system and utilizes one of the process fluid streams as a heat transfer medium to control the fuel processor.
The present invention seeks to improve the thermal management of a fuel processor by utilizing ATR process water for the thermal media. There are several advantages including a minimal parasitic pumping power requirement for the media since water can be pumped to a high pressure in liquid form, prior to its vaporization. Additionally, significant heat absorption is available with a relatively low mass flow rate by using the high latent heat energy of water. Water also has a higher sensible heat capacity and thermal conductivity compared to other known process fluids used in fuel cell systems.
The present invention is directed to a thermal management process that is adapted for use with a fuel processor for a fuel cell. The fuel processor system having an auto thermal reformer, a water gas shift reactor, a preferential oxidation reactor, a first air (ATR) stream, a fuel stream and a first (ATR) vaporized water stream. The process includes supplying the air, vaporized water and fuel streams into the auto thermal reformer (ATR). The ATR effluent is fed into the water gas shift (WGS) reactor with a second (WGS) vaporized water stream. The WGS effluent is fed into the preferential oxidation reactor (PrOx) with a second (PrOx) air stream. Control of the temperature of the PrOx is performed through vaporization of the water streams to form a first portion of vaporized water. The PrOx effluent and a third (stack) air stream are fed to the fuel cell stack. The anode exhaust stream is combined with a fourth (combustor) air stream which is fed to the combustor. The combustor exhaust heats a third vaporized water stream to form a second portion of vaporized water. The first portion of vaporized water and the second portion of vaporized water forming a steam fluid. The ATR effluent (i.e. the reformate gas exiting the ATR) gives up heat to the steam and air streams prior to entering the WGS. In this way, the temperature of the ATR effluent is conditioned for further reformation in the system, and the steam and air streams being sent to the ATR inlet are preheated to maximize reformer efficiency.
The present invention provides independent temperature control of each chemical reactor resulting in minimum reactor size and maximum performance throughout turndown and transients, with maximum utilization of waste heat for vaporization and preheating of the auto thermal reformer air, water, and fuel to minimize auto thermal reformer air requirements (o/c ratio) and thereby maximize fuel processor efficiency. In addition, the present invention accomplishes fuel processor thermal management with increased flexibility, lower mass and volume and potentially lower maintenance than a fuel processor thermal management system that uses a separate heat carrier loop (such as oil).
For a more complete understanding of the invention, its objects and advantages, reference should be made to the following specification and to the accompanying drawings.